This application claims priority to U.S. Provisional Application No. 60/894,635, filed Mar. 13, 2007 and U.S. Provisional Application No. 60/813,220, filed Jun. 13, 2007, the entire disclosures of which are incorporated herein.
This invention is generally related to power systems utilizing alcohol reforming, and more particularly, to the efficient reforming of alcohols to produce hydrogen-containing gas mixtures to use as fuel in internal combustion engines such as those used to generate electrical or mechanical power in vehicular power systems.
In transportation applications, alcohols, particularly ethanol, are garnering increased interest as an alternative to fossil fuels for internal combustion engines. Ethanol is a renewable fuel, typically derived from fermentation of agricultural biomass. Unlike fossil fuels, the carbon dioxide liberated during the combustion of ethanol does not represent an increase in greenhouse gases because the carbon atoms released during combustion represent atmospheric carbon dioxide fixed by plants from which the ethanol is derived.
However, there are difficulties associated with the use of alcohol fuels in internal combustion engines. The lower heating values of methanol (15.9 MJ/liter) and ethanol (21.3 MJ/L) are substantially less than that of conventional gasoline (32 MJ/liter) as reported by F. Black in “An Overview of the Technical Implications of Methanol and Ethanol as Highway Vehicle Fuels,” SAE Paper 912413, 1991. Thus, a greater volume of alcohol fuel is necessary if utilized with equal efficiency, which reduces the value of ethanol to the consumer on a volumetric basis.
Moreover, cold start is a problem for alcohol-fueled engines because at low temperature the fuel lacks sufficient vapor pressure to form an ignitable mixture. Anhydrous ethanol engines cannot start at ambient temperatures below about 15° C. (59° F.). Ethanol, therefore, is usually blended with gasoline in the United States (typically, 15% gasoline in E85 blend), so that the gasoline can initiate combustion in cold temperature operating environments. E85 engines can achieve cold start at low temperatures by massive overfueling in order to force enough volatile fuel into the cylinder to achieve ignition. This results in high levels of hydrocarbon and carbon monoxide emissions, a problem which is significantly aggravated by the fact that the catalytic converter is not yet at operating temperature. (See J. Ku et. al., “Conversion of a 1999 Silverado to Dedicated E85 With Emphasis on Cold Start and Cold Driveability”, SAE 2000-01-0590, 2000). Moreover, cold start problems may persist even using E85 and similar fuel blends at lower temperatures. As a solution to the cold start-up problem, G. W. Davis et al. suggest in Proc. Intersoc. Energy Conver. Eng. Con., 2000, 35, pp. 303-8 to supplement the E85/air mixture with hydrogen.
The two most important variables determining the efficiency of an internal combustion engine are the expansion ratio and the air:fuel ratio. The expansion ratio is the ratio of the volume in the cylinder at the time the exhaust valve opens to the volume at maximum compression. The expansion ratio is often, but not always, equivalent to the compression ratio. An engine's compression ratio is the ratio of the volume between the piston and cylinder head before and after the compression stroke. The air:fuel ratio is sometimes expressed as λ and sometimes as the equivalence ratio, denoted by φ. Lambda (λ) is calculated by dividing the actual air:fuel ratio by the stoichiometric ratio of air:fuel for the fuel being combusted. The equivalence ratio is calculated by dividing the actual fuel:air ratio by the stoichiometric fuel:air ratio for the fuel being combusted.
Internal Combustion Engine Fundamentals by John B. Heywood (McGraw Hill, New York, 1988) describes the effect of expansion ratio and equivalence ratio on internal combustion engine efficiency. Increasing an engine's expansion ratio improves efficiency as does increasing λ. Increasing λ above 1.0 corresponds to using “leaner” fuel-air mixtures (i.e., mixtures with an excess of air over that required by stoichiometry).
The maximum attainable compression ratio is set by the knock limit. Increasing compression leads to increased temperature and pressure of the gas in the cylinder that causes spontaneous, premature ignition known as “knock.” The ability of a fuel to resist knock is quantified by its octane number. Both methanol and ethanol are relatively high octane fuels, but methane, hydrogen, and carbon monoxide are more resistant to knock and therefore can be utilized with high efficiency in an internal combustion engine operated with a high compression or expansion ratio.
Lean combustion improves fuel efficiency in part because it ensures complete combustion of the fuel, but primarily by reducing the temperature of the combusted gas. The lower temperature reduces heat loss to the cylinder walls and improves the thermodynamic efficiency with which the gas does work on the piston. For example, J. Keller et al. report in SAE Special Publication 1574, 2001, pp. 117-22 that operating a four-stroke, spark-ignited internal combustion engine using hydrogen as a fuel under lean conditions (equivalence ratio=0.35-0.45, corresponding λ=2.2-2.9) and high compression ratio (up to 20) results in thermal efficiencies of up to 47%. A further advantage of low temperature combustion is the fact that formation of nitrogen oxides (NOx) is minimized.
When the air:fuel ratio becomes too lean (and the gas temperature too cool) the mixture will fail to ignite or “misfire.” Alternatively, the mixture may burn too slowly or incompletely. Because hydrogen will burn in air at concentrations down to about 4% and exhibits a high flame velocity, aiding rapid and complete combustion, supplementation of the fuel with hydrogen allows for reliable operation under lean conditions. As reported by C. G. Bauer et al. in Int. J. Hydrogen Energy, 2001, 26, 55-70, the burning speeds of hydrogen, methane, and gasoline in air at normal temperature and pressure (NTP) are 264-325, 37-45 and 37-43 cm/sec, respectively.
Reforming alcohols is an alternative to combusting alcohol fuels directly in an internal combustion engine. In a reforming process, the alcohol is decomposed into permanent gases that can be fed to an internal combustion engine. L. Pettersson reports in Combust. Sci. and Tech., 1990, pp. 129-143, that operating an internal combustion engine on reformed methanol rather than liquid methanol can improve efficiency. The key factors responsible for the improved efficiency are the high air:fuel ratio, the increase in the heat of combustion of reformed alcohols compared to non-reformed alcohols, and the ability to use higher compression ratios.
It is known that starting an internal combustion engine on a mixture of permanent gases produced by methanol reforming is easier than starting on liquid methanol fuel when the ambient temperature is low. For example, L. Greiner et al. report in Proceedings of the International Symposium on Alcohol Fuels Technology, 1981, paper III-50, CAS no. 1981:465116, that ignition and continuous run at −25° C. can be achieved by reforming methanol using heat from electric current provided by a battery. However, the battery quickly discharges, forcing an early and difficult transition to the use of liquid methanol fuel and eliminating any energy efficiency advantage associated with the use of reformed methanol as a fuel.
In U.S. Pat. No. 4,520,764, issued to M. Ozawa et al. and in JSAE Review, 1981, 4, 7-13, authored by T. Hirota, the use of reformed methanol to fuel an internal combustion engine at startup and during steady-state operation is reported. Engine exhaust is used to heat the methanol reformer. Using lean combustion (λ=1.7) and a high compression ratio (14), they achieved an excellent brake thermal efficiency of 42%. By comparison, the maximum value for non-reformed methanol is about 33%. Ozawa et al. report that the engine can be started on reformate (hydrogen and CO) stored in a pressure vessel.
Reformed methanol power systems tend to backfire severely if the fuel-air mixture is not lean enough because of the high hydrogen composition. L. M. Das in Int. J. Hydrogen Energy, 1990, 15, 425-43, reports that when the fuel-air mixture is not lean enough, severe backfiring is a problem for engines running on hydrogen. T. G. Adams in SAE Paper 845128, 1984, 4.151-4.157 reports that CO—H2 mixtures from methanol reforming backfire at high concentration. As a result, the rate at which fuel can be fed to the engine and the engine's maximum power are limited.
Vehicular power systems including a fuel cell fed with hydrogen to produce electrical power have also been suggested. The fuel cell vehicle may be equipped with pressurized tanks of stored hydrogen or with a fuel processor capable of converting an alcohol or other liquid hydrocarbon fuel to hydrogen. Onboard reforming of liquid fuels would enable fuel cell vehicles to achieve ranges comparable to gasoline-fueled automobiles.
Onboard reforming of liquid or gaseous fuels to yield hydrogen-containing gas mixtures can be conceptually divided into two categories depending on the temperature required. It is both thermodynamically and kinetically feasible to reform methanol to hydrogen and carbon monoxide or carbon dioxide with greater than 95% conversion at temperatures of about 300° C. A review of methanol reforming can be found in the article “Hydrogen Generation from Methanol” by J. Agrell, B. Lindström, L. J. Pettersson and S. G. Jär{dot over (a)}s in Catalysis-Specialist Periodical Reports, 16, Royal Society of Chemistry, Cambridge, 2002, pp. 67-132. Morgenstern et al. describe complete conversion of ethanol to methane, hydrogen and CO/CO2 below about 300° C. See U.S. Patent Application Pub. No. 2004/0137288 A1; and “Low Temperature Reforming of Ethanol over Copper-Plated Raney Nickel: A New Route to Sustainable Hydrogen for Transportation,” Energy and Fuels, Vol. 19, No. 4, pp. 1708-1716 (2005). Although other fuels that reform around 300° C. are known, such as glycerol, none are abundant enough to serve as motor fuels.
Most other reforming processes are highly endothermic and therefore require temperatures of about 700° C. because of the stability of carbon-hydrogen bonds in the molecule. Reforming of methane and gasoline as well as high temperature reforming of ethanol to hydrogen and carbon monoxide are in this category. Although considerable research has been devoted to onboard generation of hydrogen via high temperature reforming, fueling an internal combustion engine is not practical at high reforming temperature, largely because of the energy cost of generating the required heat by burning a portion of the fuel.
By contrast, fueling an internal combustion engine with reformed methanol is known in the art and is enabled by the fact that the reformer can be maintained at the required temperature (typically about 300° C.) by the heat of the engine exhaust. Even so, high thermal conductivity is required in the catalyst and reformer to effectively use engine exhaust as a heat source. Hirota reports in JSAE Review, 1981, 4, 7-13, that, although methanol reforming requires a temperature of only 300° C., considering the performance of the current reformer's heat exchanger, a temperature difference of about 100° C. between the exhaust and catalyst is required, so that the lower limit of the exhaust temperature is approximately 400° C. This limit corresponds to an engine speed of about 1400 rpm under no load. Thus, there are difficulties in the prior art in maintaining reformer temperature (and thus catalyst activity) when the engine is near idle.
Numerous papers have also described the high-temperature steam reforming of ethanol to carbon monoxide and hydrogen using alumina-supported, copper-nickel catalysts in accordance with reaction equation (1) below. In fuel cell power systems, it would be necessary to contact the reformate with a suitable low-temperature water-gas shift catalyst in accordance with reaction equation (2) to generate further hydrogen and eliminate CO, a fuel cell poison.CH3CH2OH (g)+H2O (g)→2CO+4H2  (1)water-gas shift: CO+H2O→CO2+H2  (2)
Reaction (1) is highly endothermic, which accounts for the requirement of reforming temperatures of about 700° C. in order to fully convert ethanol to hydrogen. The high temperature required for the reaction causes several difficulties when attempting to utilize ethanol reformed in this way for the generation of electrical or mechanical power. First, as noted above, engine exhaust is not hot enough to supply the heat required in the reformer. Accordingly, exhaust-heated, high-temperature reforming of ethanol for vehicular power system applications has not been widely developed or tested. Second, catalyst deactivation during high-temperature ethanol reforming has been reported as severe. The major cause of deactivation is coking due to the formation of polyethylene on the catalyst surface, which is converted to graphite. The dehydration of ethanol to ethylene, catalyzed by acidic sites on the support, is believed to be the root cause of catalyst deactivation. (See Freni, S.; Mondello, N.; Cavallaro, S.; Cacciola, G.; Parmon, V. N.; Sobyanin, V. A. React. Kinet. Catal. Lett. 2000, 71, 143-52.) High levels of ethylene formation have been reported on alumina-supported catalysts (See Haga, F.; Nakajima, T.; Yamashita, K.; Mishima, S.; Suzuki, S., Nippon Kagaku Kaishi, 1997, 33-6.)
Morgenstern et al. have explored fuel cell vehicular power systems fed with hydrogen produced by the low-temperature (e.g., below about 400° C.) reforming of alcohol, particularly ethanol, over a catalyst comprising copper at the surface of a metal supporting structure (e.g., copper-plated Raney nickel). Morgenstern et al. propose that low-temperature ethanol reforming may be divided into two steps, although a concerted mechanism is also possible. In accordance with reaction equations (3)-(5), ethanol is first reversibly dehydrogenated to acetaldehyde, followed by decarbonylation of acetaldehyde to form carbon monoxide and methane. After water-gas shift, 2 moles of hydrogen are produced per mole of ethanol.CH3CH2OH (g)→CH3CHO (g)+H2 ΔH=+68.1 kJ/mol  (3)CH3CHO (g)→CH4+COΔH=−19.0 kJ/mol  (4)
net after water-gas shift:CH3CH2OH+H2O→CH4+CO2+2H2  (5)
As compared to conventional high-temperature reforming of ethanol, which produces 6 moles of hydrogen per mole of ethanol after water-gas shift (reaction equations (1) and (2)), an apparent drawback of the low-temperature reforming pathway is its low hydrogen yield, producing two moles of hydrogen per mole of ethanol after water-gas shift. However, Morgenstern et al. teach that onboard a fuel cell vehicle, the methane in the reformate would pass through the fuel cell unit without degrading its performance and the fuel cell effluent may be fed to a downstream internal combustion engine to capture the fuel value of the methane (along with any residual hydrogen, ethanol and acetaldehyde). Waste heat from the engine exhaust is used to heat the reformer and drive the endothermic dehydrogenation of ethanol.
Despite the advantages provided in the teaching of Morgenstern et al. and others, the commercial development of vehicular fuel cell power systems is impeded by the complexity and high cost of the fuel cell unit as well as cold start and transient response issues. Storage of hydrogen onboard the vehicle creates safety concerns and imposes weight and cost penalties associated with the high pressure storage tanks, as well as a loss of energy efficiency caused by the necessity of compressing the hydrogen to pressures of 5-10,000 psi.
Accordingly, a need persists for reformed alcohol power systems in vehicular and other applications that use an internal combustion engine for primary power generation and effectively exploit the fuel value of alcohols with high efficiency to enable cold start-up without blending conventional gasoline and allow for leaner air:fuel operation of the internal combustion engine.